Gnome gdm vs metacity (Ubuntu)

I've structured this post in effort to promote readability by citing invocations / applications used as relevant to inquiry. The body is separated into sections indicated using ALL CAPS:

INQURY:

If i wish to configure GNOME to use "gdm", as an alternative to "metacity", what dependencies must I install (if not already present on this system)?
What manual configuration is required to achieve an operational version of GNOME/gdm, instead of GNOME/metacity (the default Ubuntu conf, apparently, since Lucid or Maverick)?MODIFIED:I see "gdm" is actually the parent process of metacity, in the gnome-system-monitor app. Am I mistaken that I've used GDM "alone" as window manager for GNOME in other Linux desktop configurations? This system is also configured to use "Openbox" as an alternative desktop session, but I would like to sample other options-- if available-- for running GNOME. The inquiry stems from my desire to gain control over such desktop window manager options, as window decoration colors, etc., which are not currently available in this default Ubuntu setup, but furthermore to learn more about personalizing the desktop, in general.

BACKGROUND:
having Linux desktop user-history dating back to FC5, i am aware of some various desktop configurations, including non-desktop window-managers.

i've learned, however, this doesn't necessarily resolve to an understanding of operational configuration in every environment, hence my resolve to inquire for help.

using this configuration, on log-in, the desktop /comes up/ , including the "panel", and applications may be invoked, yet, when attempting to configure "Windows", according to a [gnome?] pop-up dialogue, “Cannot start the preferences application for your window manager: Window manager "unknown" has not registered a configuration tool”